Introduction

This page describes the average impact of positive
Indian Ocean Dipole(IOD) events on Australian rainfall patterns. A positive
IOD year is characterised by cooler than normal water in the
tropical eastern Indian Ocean, near Indonesia, and warmer than normal water in the tropical western Indian Ocean, near Africa.
A positive IOD sea surface temperature pattern often results
in a decrease of rainfall over parts of Australia.

Eleven
years since 1958 have been identified that have general acceptance as positive
IOD years. The eleven
IOD years selected are 1961, 1963, 1967, 1972, 1977, 1982,
1983, 1994, 1997, 2006 and 2007. Maps of rainfall amounts and rainfall deciles for these and other years
are freely available from our archives.

An IOD event usually starts around May or June, peaks between
August and October and then rapidly decays. Hence the focus of this article is on the austral winter and
spring (June to November). In the last section, a display of overlapping three-month rainfall patterns
is used to show the evolution of typical positive IOD events.

Winter and spring - Positive IOD years

Figure 1 shows the mean rainfall deciles for total winter-spring (June to November) rainfall for the eleven
positive IOD years listed above. For each of the eleven years,
the rainfall deciles for the
winter-spring period were calculated against all years between 1900 and 2007. These deciles were then
averaged for each point in Australia, and the result mapped.

The map above shows that during positive IOD years, the mean
winter-spring rainfall is below average (that is, in deciles 2 or 3 and indicated by red shades on the map)
across much of central and southern Australia. It should be noted that in no part of the country is there a
consistent tendency towards above average (decile 8 or higher) rainfall in positive
IOD years.

It should not be expected that winter-spring rainfall in any given
positive IOD year will follow the pattern of Figure 1,
nor should it be expected that "above average" rainfalls will not
occur during an IOD year.
To see what happened as regards to total winter-spring rainfall in
each of these positive IOD years, click on the
appropriate year
(1961,
1963,
1967,
1972,
1977,
1982,
1983,
1994,
1997,
2006,
2007).

Winter and spring - Positive IOD and El Niño

Some positive IOD events, but not all, occur during the same year
as an El Niño.
In 2007 a
positive IOD occurred in the same year as a
La Niña.
This is the only clear occurance of a positive IOD event coinciding with a La Niña; 1967 showed a weak La Niña like
pattern in the Pacific Ocean after August, but was clearly neutral in the atmosphere.
The relationship between
El Niño and the
IOD is complicated, with the level of dependence of the two
phenomenon an area of active research.

Figure 2 shows the mean rainfall deciles for total winter-spring rainfall for seven of the recent positive
IOD years that also occurred during an El Niño event.
The seven positive IOD/El Niño years used are 1963, 1972,
1977, 1982, 1994, 1997 and 2006.

The map above shows that during positive IOD/El Niño years,
the mean winter-spring rainfall is below average across nearly all of Australia. There is a significant shift in the
pattern of Figure 2 towards dry conditions in the east of Australia.
The shift towards the east of the country is in line with the impact expected in an El
Niño year
(see map of Australian rainfall patterns during El Niño events),
however, there is still a signal of reduced rainfall across central Australia,
which is not usual when we look at El
Niño years on their own. Once again, in no part of the country is there a consistent tendency towards above
average (decile 8 or higher) rainfall.

Winter and spring - Positive IOD and neutral ENSO

Since 1958 only three positive IOD events have occurred during
ENSO neutral years.
The three positive IOD/neutral
ENSO years are 1961, 1967 and 1983.
As three years is a small sample size, mean rainfall deciles for total winter-spring rainfall has not been plotted. However, the
maps below are included so that rainfall patterns from each individual year can be viewed seperately.

The maps show that during the neutral ENSO/positive IOD
years analysed here, winter-spring rainfall patterns have similarities to that in Figure 1, with below average rainfall across central and
parts of southern Australia. Both 1961 and 1967 show a particularly strong trend of below average rainfall over central and southern Australia.
In contrast to 1961 and 1967, 1983 was wetter than average over much of southeast Australia as well as over southwest Western Australia. All three
years share in common above average rainfall along the east coast of Australia.

Evolution of positive IOD events

Figure 4 below shows the evolution of three-month rainfall deciles averaged over the eleven positive
IOD events. Each overlapping 3-month period is shown from April (April to June)
to December, to encompass a typical event from start to finish.